Atomic data for opacity calculations. IV. Photoionisation cross sections for C II

For pt.III, see ibid., vol.20, p.6399-408 (1987). The close-coupling methods described in paper II are used to calculate cross sections for photoionisation from all states of C II having effective quantum numbers v<or=10 and total orbital angular momenta L<or=3 (a total of 46 states). Most of the cross sections contain complicated resonance structures. In regions just below each new threshold, techniques of quantum defect theory are used to calculate cross sections averaged over resonances in Rydberg series. Similar techniques are used to calculate mean photoabsorption cross sections in the region just below the first ionisation threshold. For C II states of the type 2s 2 ( 1 S)nl the cross sections contain resonances due to photoexcitation of the core (PEC resonances), that is to say due to transitions to 2s2p( 1 P o )nl followed by autoionisation.